The field of this invention is keyboards and, more particularly, membrane keyboards. A membrane keyboard is a relatively thin switch array which is provided with a plastic membrane cover. The use of such a keyboard is considered advantageous where space is to be conserved or where it is desired to provide a sealed keyboard.
In the past there has been a desire to provide illumination for keyboards such as where an electrical device is to be used in low-light situations or where contrast is desired to be improved. Similarly, it has been desired to use illumination as an indicator of circuit operation or readiness. The desire for illumination has been partially addressed in some devices by the placement of a signalling lamp adjacent to the key site. This method, however, uses up available space on the keyboard, restricting design options. Moreover, adjacent placement of the illumination source does not increase legibility of the key.
Other means of illumination have offered lit switches which include phosphorescent and electroluminescent elements. These switches are typically quite dim and generally not independently addressable.
It has also been known to use a sheet of light-conducting material interposed between the contact mechanism and the actuating surface of a standard key. A single light source supplies light to translucent keys which are illuminated thereby.
A related device employs a hinged light-conducting material as a key actuator. The light of a lamp adjacent to the switch actuating mechanism is transmitted through the light-conducting material providing illumination of the switch area. These illuminating means are known to possess certain disadvantages. In particular, the use of such light conducting material occupies volume, restricts the use of materials which may be used in construction, and may restrict the placement of keys.
Further, the placement of light conducting material or any other light source, such as an LED, above the switch mechanism necessarily increases the thickness of the switch or keyboard.
It has also been known to provide a dome with a single hole through which a light source protrudes. This means of illumination is incompatible with some keyboards since the light source placement would interfere with the travel of the switch actuator. Further, in order to maintain the characteristic resilience and tactility of the typical dome switch, the illuminating hole must be kept small. Whether the light source protrudes or lies below the dome, the small diameter of this hole prevents illumination of a broad area.
The use of a clear, metallized, plastic dome has also been attempted. Such domes possess the drawbacks of high electrical resistance and low current tolerance. These metallized plastic domes also are not durable.